SU carb float level - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
User Tag List

Closed Thread
LinkBack Thread Tools
post #1 of 15 (permalink) Old 08-20-2002, 02:09 PM Thread Starter
Senior Member
Join Date: Jun 2002
Posts: 181
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
(Thread Starter)
SU carb float level

I've spent a lot of time adjusting my SU carbs (with 31 years and 128,000 miles on them). They seem to work fine as far as keeping a steady idle and delivering power. However, they suck gas, and when I punch it black smoke pours out the back. After adjusting it several times I've decide either (a) the fuel float level is too high and gas is being dumped into the carbs, or (b) the carbs need to eb replaced. In an effort not to spend $600 on new ZTherapy carbs, I want to make sure that the float level is not the problem.

My question is, if I pull the cover off of the float bowl, do I need to replace the gasket? If I need to buy a rebuild kit at $150 in order to check the float level, I might as well get the new carbs. However If I can check the float level for free, I'd rather do that before deciding the carbs are really too far gone.
ca_zcar is offline  
Sponsored Links
post #2 of 15 (permalink) Old 08-20-2002, 03:38 PM
Senior Member
Join Date: Jun 2001
Posts: 507
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)


If you remove the float bowl cover, you will certainly need to replace the gasket.
Since you say the cars are 31 years ols, the old cork gasket is no doubt going to break when removed. Heck, I've had them swell to unusable status after only 15 minutes of use.

However, you do not have to purchase new ones from Z therapy. They are not the only providers of SU parts. Nissan still carries the gaskets, as do most auto parts stores. However in the case of your local auto parts store you will have to buy the rebuild kit which will include some, but not all of the parts in the Ztherapy rebuild kit.

If I were you, I'd consider going to the local autoparts store, getting some cork gasket material and an razor blade and trying to make myself some gaskets. This would cost about $5 for all of the float bowl gaskets you could ever want.

Just my $.02
Loosetoe is offline  
post #3 of 15 (permalink) Old 08-20-2002, 06:47 PM
Senior Member
Join Date: Jun 2001
Posts: 320
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Re: gasket

hello ive removed the covers and checked the float level many times on my 240...and had no problem with just making a new gasket ..ive only had to make them once but it's not that i've never seen the S.U. carbs with cork for the float bowl gaskets..infact i dont think that cork will hold up against the harshness of gas...i would not use seems to me that it would be asking for trouble using cork on the float bowls..cork tends to break off in small pieces and this will surely clog the float needles eventually...infact there are people that dont run with gasket's on the float bowls at all...the only time the gas would ever get close to the top of the float bowl enough to leak would be on very sharp turns...otherwise the fuel should never reach that high in the bowl...there are threads on the search that tell how people have made float bowl level sites..using a clear plastic tube connected to the drain plug of the float can check that out...but dont spend all that money to check your floats..and if your carbs are running ok untill you step on it..i would suspect that you just need to go through and check the float level. the needle setting in the carbs..and adjust the carbs...atleast try that before spending big bucks on new stuff...good luck michele
mizhele is offline  
post #4 of 15 (permalink) Old 08-20-2002, 09:42 PM
Junior Member
Join Date: Jun 2007
Posts: 15
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Re: SU carb float level

Hi my name is joel and ive been driving a z for a long time maybe i can help. as far as your floa levelare concerned remove the float allembly and invert it blow thruthe inlet tubes and with the weight of the floats resting on the needle you shouldnt be able to blow thu the inlet. now while raising the float upward while in the inverted pasition determine that you are able to pass air thru the inlet only when the top of the flaot which now appears to be the bottom since it is inverted reaches just upwards of the level position. Ive made this adjustment many times without replacing the gasket just be gentle. as far as the black smoke this is sometimes caused by too thin of oil in the dampener in the top of these carbs.I woul suggest removing the oil you have in them now by suction a syrenge and a peice of vac hose works well. after you have evacuated the oil spray some carb cleaner inside thes compartments and work your slides up and down few times this cleans them . now remve the caleaner and install some twenty weight oil or dextron automatic trans fluid in the carbs. also before you screw the dampener pistons back into theyre spots try them in alternate spots reversing their positions. install them in the spot that offers the most resistanc when you push them in and out of their bores .primarily email add. is fairldykat @aol,com
fairldykat is offline  
post #5 of 15 (permalink) Old 08-20-2002, 10:18 PM
Senior Member
Join Date: Jun 2001
Posts: 507
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)

To respond to your comments about the cork gasket.

New gaskets purchased from wither Nissan or Ztherapy are made of cork, albeit very thin cork. This is why I recommended using it.

As a counterpoint to your statement that you've never seen a float bowl with a cork gasket, well...I've never seen one without one. I guess we'll have to agree to disagree on this one.
Loosetoe is offline  
post #6 of 15 (permalink) Old 08-20-2002, 10:26 PM
Senior Member
Join Date: May 2002
Posts: 835
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Re: SU carb float level

Hmmm if they seem to run fine during normal driving but run very rich if you suddenly floor it I would say that maybe you have the wrong weight oil in your carbs' dashpots. If you're car is relatively stock you should be running ATF or Marvel Mystery Oil in the dashpots. Heavier oil will cause the pistons to raise much slower... which when flooring it at low revs will cause the engine to pull down a lot of fuel until the piston opens up and isn't blocking the airflow anymore.
You may also want to check the needle position in the piston, make sure it's set properly. You should be able to completely close off the fuel supply by raising the mixture nuts all the way up. Many times they have been adjusted using a straight edge but this doesn't get them perfect and even with mixture nuts all the way up there is still a little between the shoulder of the needle and the top of the fuel nozzle. If you haven't seen the Ztherapy video, I'll descripe the best way to do this. Take off the dome covers on the carbs, one at a time, DON"T MIX UP the pistons, and their respective domes and what carb it came off of. While you have them off take a good look at the needles... are they bent or do they look very worn? Loosen the lock screw that holds the needle in place, and pull it out like 1/4 inch.. or whatever distance but just pull it much farther than it should be sticking out. Then tighten the lock screw only a very little bit, just enough to hold it in place. Screw the mixture nut on the carb body all the way up/lean. Put the piston back in place, then forceable push the piston down and the fuel nozzle up simultaneously. This will perfectly set the needle to where it will cut off fuel supply. Take the piston back out and fully tighten the lock screw, and then put everything back together. I had this problem on my old SU's where the needle was so recessed into the piston that even with the mixture nut all the way up the car still ran very very rich. Someone wasn't paying attention to what they were doing.... By the way when you check the float level be sure that the needle valves are also tight in the float lid, sometimes they come loose a bit and leak, causing rich running. Make sure they're not sticking and that the needle can be moved easily. Hope all this helps.

'72 240Z, rebuilt stock '81 ZXT motor, 1G DSM BOV, DIY MBC, 10psi, Buick GN I/C (soon), K&N filter, 2 1/2" crushbent exhaust, glasspack, short shifter, 16lb flywheel, ACT 6 puck, '82 5 spd, 3.54 diff, Eibach's, Tokico's
bastaad525 is offline  
post #7 of 15 (permalink) Old 08-20-2002, 11:22 PM
Senior Member
Join Date: Apr 2001
Posts: 347
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Re: SU carb float level

First of all if you can pull the gasket off without replacing it if it not too old. I've pull mine off many times and its fine and yes its cork. You will fine a ton of information regarding adjusting the floats in the archives. If you already search there then check your spelling and search again, it been covered.
WoZZd is offline  
post #8 of 15 (permalink) Old 08-21-2002, 12:09 AM
Senior Member
Join Date: Nov 1998
Posts: 197
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Re: SU carb float level gaskets in

Those carb "refresh" kits (for that's all they really are anyway...) you get at NAPA, Advance, Pepboys; include all the gaskets you need for a single carb.

IIRC, they are under $20.00USD per carb.

Those thin, fragile float-bowl gaskets would be a real challenge to carve out of paper or index card with an Exacto knife.. Good luck!

As to your original question, the instructions with the rebuild kits and my Haynes and Clymers all say 0.55" - 0.59", or 9/16" from top surface of float toe edge to float-bowl cover surface. The 70-71 carbs were advised to be 13/32nd".

The Scott Bruning video has a more seat of the pants variation that also works; basically hold your float cover with float attached upside down, and blow upwards thru the fuel feed pipe, fiddle with the float tab until you can stop the air flow thru the needle/seat with the float tab and float about level. It's a visual thing.
Skip is offline  
post #9 of 15 (permalink) Old 08-21-2002, 02:40 AM
Senior Member
Join Date: Mar 2000
Posts: 4,236
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
how to tune su's

SU Carburettors
[Did you know that S.U stands for Skinners Union. The Skinners are the brothers who turned a family leather business into making very successful carburettors. The S.U carburettors have figured in a large number of vehicles from the beginning of this century to the early 1980's. The earliest S.U's had leather bellows instead of the piston to slow the rise of the air-flow....... Salman SHAMI ]

1. Tuning your SU carburettors by Scott Fisher,

2. Tuning Your S.U. Carbs by Roger Garnett


1. Tuning your SU carburettors by Scott Fisher,
I've been meaning to write this up for some time, ever since I did the SU Performance Tuning 101 a few months ago. This one is more like Basic SU Adjustment for Happy Driving.
The trick to tuning SU carbs is to understand that there are two things you need to get right: the air flow, and the fuel mixture. While they are interconnected, they are also independent, and need to be measured and adjusted independently.

Special Tools

You will probably need to arrange to buy or borrow a Unisyn flow meter. The Unisyn is the usual gauge for getting the air flow balanced between the two carbs. This costs about $20 and is simple to use. It consists of an adjustable opening (same size circumference, but with a disc on a threaded rod that you can screw tighter or looser) that you use to set the level of a little float that rises or falls in a glass tube at the side of the gauge.

For the fuel mixture, I have become sold on a device called the Gunson ColorTune (maybe ColourTune, as it's a British co.). This is a spark plug with a crystal pressure- and heat-resistant window in it that lets you see into the combustion chamber while the motor is running. The color of the flame indicates the mixture richness. It costs about $40, and while it's not absolutely essential, it makes life so much easier that it's worth the cost.

If you don't have a Gunson, I've included the standard directions here for determining correct mixture (step 4 of the Adjusting Mixture procedure).

To tune SU carbs, first locate the following components:

Throttle linkage nuts. These are the things that connect the throttle linkage (the bar connected to your foot through whatever means your car uses, cables or rods) to the carburetors' throttle levers.

Throttle stop screws. These set the idle speed for each carb, and are located typically behind the dashpot, on the same side of the carb to which the throttle linkage connects.

Mixture adjusting nut. This is the lower of the two nuts at the very bottom of the carburetor. Later SU carburetors of the HIF type have integral float chambers, on which the mixture is adjusted by turning a screw. You'll need to experiment (and I explain how) to see which way makes this richer and which way makes it leaner.

Lifting pins. These are little wobbly metal pins under the dashpot. When you push up on the pin, it raises the piston in the dashpot. Find these; they're crucial if you don't have a Colortune. If you don't have or can't find them, you can raise the piston with a flat-bladed screwdriver pushed down the throat of the carb and twisted to lift it.

The bridge. This is the part inside the carburetor, where the gas jet opens into the airstream. You'll see a needle inside the jet, and the jet itself should be a few fractions of an inch down from the bridge itself. The jet is the brass tube that sits in the center of the bridge, with a tapered needle poking down into it.

The choke linkage nuts. Comparable to the throttle linkage nuts (and usually the same size), but on the linkage that goes between the choke cable and the mixture adjustment mechanism. They make sure that both carbs are enriched when you pull on the choke.
Balancing The Air Flow

1. Start with the engine warmed up to operating temperature and perform your standard ignition tune-up (points gap, timing, spark plug gap, new condenser, etc.) first. If you've got a timing light and a dwell meter, you can verify all that stuff independent of the way the car is running. When it's warm, shut the motor off and remove the air filters.

2. Begin by balancing the air flow. To do this, first loosen the throttle linkage nuts. Leave them connected, just loosen them half a turn or so.

3. Back out the throttle stop screws till you can see that they are just touching the throttle stop. Then open each carburetor (that is, lower the throttle stop screw) 1-1/2 turns of the throttle stop screw and start the engine. It will probably idle at about 2000 RPM; don't worry.

4. Put the Unisyn over either carb and adjust the orifice in the Unisyn till the little float at the side rests at the middle of its graduated tube. (Pre-diagnostics: if the idle drops and the car wants to die when you slap on the Unisyn, the carb is too rich; if the idle soars upwards, it's too lean.) Hold the Unisyn over the carb for only long enough to see the level of the float, then remove it.

5. Place the Unisyn on each carburetor in turn to check its flow, adjusting the throttle stop screws until both carburetors register the same position on the graduated tube of the Unisyn. (The float will probably move either up or down in the tube, which is why you want to center it in Step 4.) When both carburetors flow the same amount of air, tighten the throttle linkage nuts, adjusting for the amount of free-play between the linkage and the throttle stops that your manual calls for (probably about 0.006"). Your goal should be to achieve the lowest possible idle with both carbs balanced and the engine running smoothly. (Note that the idle speed will very probably rise as you get the mixture correct.)

If you've taken more than five minutes to do this, rev the engine to over 2500 RPM (assuming the idle isn't already that high) for thirty seconds or so to clear the spark plugs. Then adjust the mixture.

Adjusting The Mixture:

Note: in the following procedure, one "flat" is the basic increment of adjustment, and refers to 1/6 of a turn of the mixture adjusting nut. This corresponds to the flat faces on the nut.

I'm going to give instructions for SUs with the separate float chambers. If you have the HIF integral-float carbs, you'll have to look in a manual to see whether you turn the mixture screw to the right or the left to make it richer or leaner; I've done that once but I can't remember. Alternatively, you can -- with the motor shut off -- peer down the throat of the carb and turn the mixture screw while watching the top of the jet. Remember that moving the top of the jet up will lean out that carb, while moving the top of the jet down will richen it.

1. Shut the car off and loosen the choke linkage nuts.

2. Adjust the mixture nuts (screws) fully lean.

For separate float-chamber cars, this means raising the mixture nut all the way up against the bottom of the carb (or rather, against the spring). For HIF carbs, you can try turning the screw while looking down the throat to see which way the jet is moving. In either case, the idea is to zero out the jet: raise it all the way up in the bridge.

3. Now drop the jet an equal amount -- two full turns for HS-type carbs, two full turns (I believe) for HIFs. Then start the car.

Note: In the following step, you might want to consider adjusting the carburetors one-half a flat too lean, as the mixture will be enriched when you put the air filters (which restrict air flow) on at the end of the tuning process.

4. Raise the lifting pin (or use a screwdriver if you don't have the pins) so that the piston rises no more than 1/16". Listen to the engine's exhaust note and compare it to the following conditions:

If the exhaust note rises and stays high till you drop the piston, this carburetor is adjusted too rich. Turn the mixture nut one flat (one-sixth of a turn) up, moving the jet toward the bridge, then repeat Step 4.

If the exhaust note falls and the car sounds as though it is going to stall, this carburetor is adjusted too lean. Turn the mixture nut one flat (one-sixth of a turn) down, moving the jet away from the bridge, then repeat Step 4.

If the exhaust note rises briefly and then settles back down to something like the original RPM level, this carburetor is set correctly. When you have achieved this setting for both carburetors, continue with Step 5.

5. Tighten the choke linkage nuts so that the choke cable will pull an equal amount on both mixture nuts when you pull the knob.
6. At this time, I find I usually have to adjust the idle again because getting the fuel mixture right usually changes the idle speed. Since you know you have the throttles synchronized, I normally just adjust the idle without loosening the throttle linkage. The easiest way is to screw one of the screws out till it doesnt' even touch the throttle stop, then use the other to get the idle speed right. When that's done, you can screw the other stop screw down till it just touches the stop on that carb and you're set.

7. Replace the air filters and go for a test drive!


SU carburetors are most fuel-efficient when slightly lean, and provide the most power when they are slightly rich. You can use this knowledge to provide a certain amount of tuning for the kind of driving you do. If you learn to read spark plugs, you can get a basic idea of what your engine's condition is and make fine adjustments to the mixture nuts accordingly.

If you have a ColorTune, you simply install it in place of one of the plugs, then adjust the carburetor that feeds that cylinder (the front carburetor for 1 & 2, the rear for 3 & 4). The ColorTune will let you see the color of the flame. White flashes mean too lean; yellow flame means too rich. Blue (like a Bunsen burner) is correct, and blue with a faint orangish tinge is the best for power.

You can also modify your car's throttle response characteristics slightly by adjusting the viscosity of the oil in the dashpot damper. SUs are set up so that a thicker oil will resist the piston's attempt to rise in the dashpot for just long enough that the engine's increased load (when the throttle is opened) will pull more fuel across the bridge; this enriches the mixture and temporarily bumps power up to help the engine achieve higher speed more readily.

If you modify your engine, you will probably need to modify your needles, as it is the needle profile that determines the mixture curve for different air-fuel loads.

If you experience uneven idle, hunting, or an idle that changes (rises or falls) as the engine's temperature climbs or drops, you probably have vacuum leaks. The most serious fault on most old SUs is wear in the throttle shaft area. To test for this, spray some carburetor cleaner on the outside of the throttle shaft; carburetor cleaner is non-combustible, and if the engine speed drops, it means some of this is getting into the air stream from outside the carburetor. You may also have leaks from the manifolds, from tubing such as the vacuum advance line to the distributor (if fitted), or from other places; the carb cleaner trick works well for locating those leaks as well.

Other problems that SU carbs experience involve dirt in the dashpot and occasionally in the float chamber. The dashpot is a precision piece of machining that involves very close tolerances so that the piston doesn't stick or bind when it rises and falls. A little grit between the piston and the dashpot can make the car jerk and sputter. Take the dashpot off, wipe the insides down with carb cleaner and a lint-free, clean rag, then reinstall it, getting the screws down tight. Also, don't swap the pistons between dashpots; they're matched to one another so that the clearance between the piston and the wall of the dashpot makes a tight seal but permits easy rising and falling.

Dirt in the float bowl basically shuts off that carburetor (or can make it flood open, depending on whether the dirt is wedging the valve open or closed). You can try rapping on the float bowl with the handle of a screwdriver, but your best bet is to take the cover off, clean out the valve fittings, and reinstall everything, with a new fuel filter for good measure.

Some older SU models also have adjustable floats, in which you need to set the float height (which basically equals the fuel level in the float chamber) by bending a brass rod. These carburetors were replaced in the mid-1960s with carburetors that had fixed, plastic floats which are basically trouble-free unless abused. The stop at the back of the floats can break if they are installed badly, and the brass pin that holds them in place can wear an oval hole in the float pivot. New floats are fairly inexpensive and aren't a bad idea if you're doing a rebuild.

Grose-Jets are very popular with some people and a big pain for others. It appears -- and this is just conjecture -- that Grose-Jets work best in cars with adjustable floats, as they are longer than the stock SU float valves. The standard failure for Grose-Jets is to flood the carburetor. I have never had problems with the stock SU float valves or floats.


2. Tuning Your S.U. Carbs by Roger Garnett
Well, it's not really that hard to set up SU's, just different. Of course it always gets more interesting when you have more than one... There is a very good Haynes SU carb manual available, reccommended reading. The basic syncing process also applies to Zenith-Stromberg's, but the adjustment mechanisms are different. Here is a laymans guide to adjusting SU's (long):

step 1-

Tune up the rest of the engine- REALLY! clean or replace, and set the points, set the timing, plugs, valve lash, and remove the air filters. (have new ones ready) All of these things can affect the setting of the carbs, which should be done LAST, (if at all). The carbs rarely need to be adjusted, once set. Also replace/install the gas filter. Of course, it helps if the carbs are in good mechanical condition as well. But you can consider a rebuild once you have gotten things working first!

step 2-

clean the carbs! use gum-out or similar stuff, clean all external linkages, shafts, and stuff.

step 3-

Remove the float bowl covers, clean the float bowls, remove old sediment, and check/adjust the float setting. (turn the cover upside down, and get a *1/8" in drill bit, set the drill bit accross the cover, the float tab should just touch the bit.) Make sure the needle is moving and seating properly. This is just like *most* floats. Replace the cover.

* This is for HS4 SU's- (1/8-3/16") if you are dealing with 1", H's, HS2's HS6's, HIF's, etc.- check the spec for your carb.

Note: You can check for matching float settings, after setting the mixture, by removing the pistons, and peering down at the jets. The fuel level should be about the same on both carbs, a little below the top surface of the jet. (After car has been run only)

step 3b- Go get a pint of ale, or something close, and set it nearby.

step 4-

Remove the piston covers. CAREFULLY remove the piston, DO NOT BEND THE NEEDLE. Set the piston down on a clean wadded rag to prevent rolling. Clean the inside of the carb. Check operation of the throttle. Check the throttle shaft slop- this is the most common place for wear on an SU, and is often where air/vacumn leaks occur. The bushings and shafts can be replaced, but it requires some machining. A small amount of leakage can be tolerated, the car just won't idle as evenly. Clean the piston. Stare in awe at the odd carborator design, simple and effective, (constant velocity). Dump the old oil out of the damper if you haven't already spilled it. clean. Reassemble, check piston movement, raise it, then let go, it should fall freely. If not, check assembly again, make sure the piston isn't binding against the carb body, it should ride only on the damper shaft. Do not strech the spring. When all is operating properly, fill the damper with Marvel Mystery Oil for light damping, or use motor oil for heavier damping. (I use MMO) If you get "flutter" on, acceleration, you might try the heavier oil.

step 5-

Start the car and warm it up, then turn off/disconnect/otherwise disable the choke mechenism. (Loosen the nuts on the clamps so that the choke stuff isn't doing anything) This will get set later. (Later Zenith-Strombergs have a thermostatic choke, not a cable.)

step 6-

Check coarse throttle adjustments- make sure the throttle cable pulls on both carbs equally, and returns completly when released. This is adjusted by loosing the set screws on the throttle shaft and matching the two sides. You can also adjust the cable length at this time, using the cable set screw/retainer at the end of the cable. You can check the float adjustments now by removing the piston & cover, and looking at the fuel level in the needle seats. Both carbs should be about even, a little below the top surface of the jet. If not, readjust one or both floats to match the level.

step 7-

Syncronize the throttles- if you have a uni-syn, here's your chance to use it, (or other air flow guage), if not use a tube and listen to the airflow. The Uni-Syn is much easier to use, and can result in better balance. Alternatly adjust the idle screw on each carb, attempt to set the idle as low as possible (~800-1000 RPM). Adjust until the airflow is *close* to the same at each carb. The engine may now be running rough, just keep the idle speed high enough to keep running. Give the throttle a quick snap to make sure everything is settled, then check sync again. Periodicly snap the throttle to make sure everything is seated. Large differences in where you can adjust the two carbs may indicate air/vacumn leaks, or other problems, such as a bad valve)

Magic Time- Relax, and shake your voodoo rattle...

step 8-

Adjust the mixture- this is done with the spring-loaded hex fitting under the carb, where the fuel supply tube enters from the float. Turning the fitting raises and lowers the needle seat. Pick a carb, and turn the fitting 3 flats (1/2 turn), first in one direction, then back 3, then 3 in the other direction. Note where the engine runs better, idle speed should increase. Turn to the best setting. Repeat this proceedure until you get the best operation you can, (higest idle speed), keeping track of flats turned will help you remember where you were. If you get lost, turn all of the way in, then back out 12 flats and start again. Periodicaly snap the throttle and push up on the fitting to make sure everything is seated. Note: Type HIF carbs (With integrated float bowl) no longer have the hex nut to adjust the mixture. Instead, there is a screw to twiddle, on front of the front carb, and behind the rear. The screw is connected to the needle seat through a temperature compensated gizmo, which is said to make the carbs more stable. Adjustment can be done in much the same way, by counting turns/flats of your screwdriver. There is less adjustment range than with the the basic models. When you think you're close, stop, uncramp your fingers, breath deep, and do the same to the other carb. Then retune the first carb, and then the second again. This serves to match the mixture of the 2 carbs, and prepare you for the beer sitting over there in the sun. (why do you think the British drink warm beer?)

step 9-

repeat step 7, setting the idle speed as low as possible, and re-syncing the idles. Now go back and readjust the mixtures. After a couple of iterations, the engine should be running smoothly (controlled by mixture) and at a low idle. Repeat as necessary. Set the final idle to 800-1000 RPM, depending on the condition of the rest of the engine.

This is a standard mixture test, performed AT IDLE: Under operation, (air filter off) lift the carb piston by 1/16" with the lifting pin or a screwdriver, which leans the mix a tad. If: -RPM's rise and stay up, that carb is rich. -RPM's rise briefly, then drop, mix is about right. -RPM's fall, engine gets rougher- mix is lean.

[where is step 10--Ed]

step 11-

Adjusting the choke- I won't get into the temperature compensation in the type HIF, or the Thermostatic choke in the later strombergs. Check the manual for more info. The choke is supposed to do two things; the first half of travel moves a cam on each carb which opens the throttle, for warm up. The second half pulls down on the needle seat to enrichen the mixture, for starting. Start with the choke in the off position (knob in). Adjust the so that the cam only starts moving the throttle after you start pulling out on the cable (adjust with shafts and adjusting screws). Try to get both carbs adjusted the same, so that both screws begin to hit the cam at the same time. This is not real critical, but you can use your Uni-Syn to match air-flow on both sides, with the choke partly engaged. After the cable is about halfway out, it should start engaging the lever which pulls down on the needle seats. Adjust the linkages so both carbs are acted on equally. You can do this by adjusting for even running of the engine. Of course, for a warm engine, the richness of this mixture will cause some roughness. Make sure the needle seats return freely when you release the choke.

step 12-

Drink that warm beer (only one, no DWI now...) it will taste great at this point!, go wash up, and go for a ride.

Notes: These procedures assume that your engine/carbs are in reasonable operating condition. If something is malfunctioning/leaking, etc, this should still help, but the results may vary. For instance, if you have leaky carbs, worn needles, engine modifications, etc, you may find things work better if you tune for optimum performance at open throttle rather than idle.

The first time through carb adjustments can be confusing, once you've done it, all of the stuff in the manuals makes sense. Go back and read them again- As always, I reccommend Bentleys, which is a repro of the original factory manuals, and then Haynes, and throw out the Chiltons. (orginal factory manuals are to be read in a clean enviroment, repros are for smearing grease all over, except, if that's all you got, use it!) Haynes has an excellent manual just for SU carbs, it covers operation, theory, rebuild of all models, and has needle charts for hundreds of car/engine/carb setups. They also have a manual for Zenith-Strombergs, which, while similar, are a whole 'nother beast. -Saftey Fast!


My drinking team has a racing problem

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

if you are fully in control, you are not driving fast enough!

79 ZXR 406 SBC 532HP 745NM 6 speed caged lsd coiled etc etc
78 ZX isma Widebody jackstand (project eternity)
E36 GTR full race M5 v8 money pit
frank280zx is offline  
post #10 of 15 (permalink) Old 08-21-2002, 03:27 AM
Senior Member
Join Date: Feb 2001
Posts: 1,713
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Forum Moderator, TAKE NOTE !

You guys should copy Frank's post to the ARTICLES section for easy future reference. (If it aint copyright infringement?)

Even if few Z's have seen an SU HIF type and front carb feeds 12&3, etc.

Thanks Frank for a great read!

Good luck ca_zcar! Ya are gonna need to spring a few bucks here and there for tools, parts. FSM ($50?) and Ztherapy video ($15) would be excellent investment. Compare that to two hours tuneup at any garage at $75/hr.

Al 260Z is offline  
Closed Thread

Quick Reply

Register Now

In order to be able to post messages on the Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page

  Similar Threads
Thread Thread Starter Forum Replies Last Post
Carb float level confusion midnightmoonlight 70-83 Tech Discussion Forum 8 10-15-2006 10:19 PM
SU carb float level ca_zcar 70-83 Tech Discussion Forum 0 08-20-2002 02:07 PM
How do I set my float bowl level? Andrew240Z 70-83 Tech Discussion Forum 2 12-03-2001 05:29 PM
float level Question ZCARBILL 70-83 Tech Discussion Forum 2 07-13-2001 06:06 PM
How do you set float level? Unknown User 70-83 Tech Discussion Forum 2 04-10-1999 11:09 PM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

For the best viewing experience please update your browser to Google Chrome

Content Relevant URLs by vBSEO 3.6.1